In order to measure crack initiation properties of materials, test specimens are provided with deep notches having a tip region. Such test specimens are then subjected to tension to measure crack initiation properties such as fatigue, corrosion fatigue, and stress corrosion cracking. However, the machining process used to provide the notch in the test specimen induces residual stresses and disstressed metal in the form of a surface machining affected zone or layer. Such a surface machining affected zone or layer has a variable and confounding effect on any test results involving crack initiation properties of materials.
It is known that the surface affected zone can be removed by an anodic dissolution using electrochemical polishing techniques prior to tensioning of the test specimen. This is accomplished by using a bulk circulating electrolyte and large surface electrodes remote from the notch and tip where it is desired to remove the surface machining affected zone. While such a process does remove some material from the deep notch tip region, it also causes metal removal from all of the exterior test specimen surfaces as well. In addition, such a process does not facilitate a controlled, uniform surface removal from the deep notch tip region. Although a stop-off agent on the exterior specimen surface can be utilized to prevent general surface dissolution, any dissolution of the deep notch tip region still remains irregular due to the remotely placed electrode and the test specimen geometry induced current gradients in the electrolyte.
Conversely, local electrochemical surface processes must avoid non-uniform surface conditioning, such as test metal grain boundary attack or metal surface microcracks. Such conditioning would compromise definition of the crack imitation phase of material behavior.
Various electrochemical machining processes and apparatuses have been disclosed in the prior art. For example, in U.S. Pat. No. 3,793,169 (Joslin), an electochemical machining process for machining small deep holes is disclosed. This machining operation is accomplished using a thin hollow cathode which conducts the electrolyte to the work area through a central bore. The effluent electrolyte flows away from the work area in the annular space created between the surface of the hole being drilled and the hollow cathode. Another electrochemical treatment of small holes is disclosed in U.S. Pat. No. 3,816,272 (Joslin). According to the disclosure of this patent, the recast surface layer of a laser-drilled hole is removed by an electrochemical machining technique which involves the positioning of an electrode in the hole and the flowing of electrolyte through the hole and around the electrode.
An apparatus for electrolytic polishing is disclosed in U.S. Pat. No. 4,431,501 (Leppanen). The disclosed apparatus includes a soft surface supported on an arm with conductive members in the soft surface. The soft surface is an absorbent material containing an electrolyte. In U.S. Pat. No. 4,125,444 (Inoue), an electrochemical polishing method is disclosed which makes use of an electrotool having a hollow handle through which electrolyte is pumped to a specific area to be polished.